In the conventional art, there has been developed an oxygen sensor, which is provided with a detecting element having a detecting electrode and a reference electrode of platinum acting as an oxidation promoting catalyst formed on the outer wall and inner wall of a solid electrolyte (as will be called the “substrate”) of a cylindrical shape having one end closed, so that it may detect an oxygen concentration on the principle of an oxygen concentration cell. This oxygen sensor is attached to the internal combustion engine of an automobile or the like so that it may be used for grasping the combustion state (or the A/F ratio) of the internal combustion engine.
Here, the detecting electrode of the detecting element in that oxygen sensor is formed through a nucleus depositing step of depositing the nuclei of platinum on the surface of the substrate and an electroless plating step of growing the deposited nuclei by an electrolessly plating method or an electrically plating method (as referred to JP-B-62-56978 (the term “JP-B” as used herein means an “examined Japanese patent publication”), for example).
First of all, at the nucleus depositing step (corresponding to the active point forming electrolessly plating step in JP-B-62-56978), the nuclei of platinum are deposited on the outer wall of the substrate by dipping the substrate in a container containing an aqueous solution of platinic ammine and by adding to this aqueous solution a reducer of sodium boron hydride (SBH) having a high reducing power. At this nucleus depositing step, however, at the time of dipping the substrate in the aqueous solution of platinic ammine, the portion of the outer wall of the substrate other than the desired one is coated with masking rubber so that the nuclei may not be deposited on the undesired portion.
At the end of the nucleus depositing step, moreover, the substrate is taken out from the container, and the masking rubber is removed. The substrate is rinsed to clear the outer wall of the substrate of the platinic ammine and the sodium boron hydride, and the process transfers to the nucleus growing step.
At the nucleus growing step (corresponding to the thin film electrolessly plating step and the thick film electric plating step in JP-B-62-56978 (pp. 3 to 4, FIG. 2)), like the nucleus depositing step, the substrate is dipped in the container containing the aqueous solution of platinic ammine, and a reducer of hydrazine having a weaker reducing power is added to that aqueous solution so that the nuclei deposited on the outer wall of the substrate may be gently grown to form the detecting electrode on the outer wall of the substrate. Here, at the nucleus growing step, the substrate is dipped in the plating liquid without being coated with the masking rubber.